Software defined radios offer the promise of wider capability in a smaller hardware footprint. This has become more important in cellular handsets as customers demand a wider variety of radio capabilities in a single mobile terminal while still expecting it to fit neatly in a pocket. Such multi-band radios operate over a wide range of frequencies to implement multi-mode and multi-band radios for cellular and data connectivity applications. However, current technological limitations have prevented realization of widely tunable radio-frequency (RF) front-end circuits for multi-band radios. Such multi-band radios are increasingly required by certain cellular radio access technologies (RAT) such as evolved Universal Terrestrial Radio Access Network (E-UTRAN, also known as long term evolution or LTE).
In the typical RF front-end, the first component after the antenna is the multi-band switch/diplexer followed by the duplexer. The duplexer is used to isolate the transmitted and received signal for the antenna. The transmit signal has a very high power, up to 33 dBm, while receiver is required to pick up signal as low as −109 dBm. Therefore, the transmit power has to be isolated from the receiver even though it is operating in a different frequency band. The transmit and receive frequencies are typically not widely separated so about 50 dB of isolation is required. This is usually done by using a surface acoustic wave (SAW) filter that operates over a limited band of frequencies. While a SAW filter may be sufficient for limited bands in for example UTRAN, the inventor considers it too cumbersome, costly, large, narrowband and inefficient (insertion loss of 2-3 dB directly impacts receiver sensitivity and noise figure) for use in more advanced RATS like LTE.
Other relevant teachings may be seen at U.S. Pat. No. 3,603,898 by John H. D. Chelmsford et al.; UK Patent 1,341,182 by Michael A. Kaufman; and a paper by Milad Darvishi et al. at section 21.1 (Analog Techniques, pages 358-359) of 2012 IEEE International Solid State Circuits Conference.